Rev 1; 2/09 EVALUATION KIT AVAILABLE General Description The is a single-chip modem with Highway Addressable Remote Transducer (HART) capabilities and satisfies the HART physical layer requirements. The device integrates the modulation and demodulation of the 1200Hz/2200Hz FSK signal, has very low power consumption, and needs only a few external components due to the integrated digital signal processing. The input signal is sampled by an analog-to-digital converter (ADC), followed by a digital filter/demodulator. This architecture ensures reliable signal detection in noisy environments. The output digital-to-analog converter (DAC) generates a sine wave and provides a clean signal with phase-continuous switching between 1200Hz and 2200Hz. Low power is achieved by disabling the receive circuits during transmit and vice versa. The is ideal for low-power process control transmitters. Applications 4 20mA Loop-Powered Transmitters for Temperature, Pressure, Flow, and Level Measurement HART Multiplexers HART Modem Interface Connectivity Pin Configuration HART Modem Features Single-Chip, Half-Duplex, 1200bps FSK Modulation and Demodulation Digital Signal Processing Provides Reliable Input Signal Detection in Noisy Conditions Sinusoidal Output Signal with Lowest Harmonic Distortion Few External Components Enable a Space-Saving Solution Standard Component 3.6864MHz Crystal Complies to HART Physical Layer Requirements 2.7V to 3.6V Operating Voltage 285µA (max) Current Consumption Space-Saving, 5mm x 5mm x 0.8mm, 20-Pin TQFN Package Ordering Information PART TEMP RANGE PIN-PACKAGE -JND+ -40 C to +85 C 20 TQFN-EP* +Denotes a lead(pb)-free/rohs-compliant package. *EP = Exposed pad. TOP VIEW 17 18 19 20 AGND 1 2 REF AVDD 15 14 13 12 11 16 10 XCEN + *EP 3 4 5 OCD 9 8 XTAL2 7 XTAL1 6 RTS *EXPOSED PAD. THIN QFN (5mm 5mm) Maxim Integrated Products 1 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim s website at www.maxim-ic.com.
ABSOLUTE MAXIMUM RATINGS Voltage Range on All Pins (including AVDD, ) Relative to Ground...-0.5V to +3.6V Voltage Range on Any Pin Relative to Ground Except AVDD,...-0.5V to (V + 0.5V) Operating Temperature Range...-40 C to +85 C Storage Temperature Range...-65 C to +150 C Soldering Temperature...Refer to the IPC/JEDEC J-STD-020 Specification. Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. RECOMMENDED DC OPERATING CONDITIONS (V = V AVDD = 2.7V to 3.6V, T A = -40 C to +85 C.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Digital Supply Voltage V 2.7 3.6 V Analog Supply Voltage V AVDD V AVDD = V 2.7 3.6 V Ground GND AGND = 0 0 V Digital Power-Fail Reset Voltage V Monitors V 2.59 2.64 2.69 V Active Current I DD V AVDD = V = 2.7V (Note 2) 285 μa Input Low Voltage V IL 0.30 x V 0.75 x Input High Voltage V IL V V V Output Low Voltage V OL I OL = 4mA 0.4 V 0.8 x Output High Voltage V OH I OH = -4mA V V I/O Pin Capacitance C IO Guaranteed by design (Note 3) 15 pf Pullup Resistance R 19 45 k Input Leakage Current XTAL, I ILRX -30 +30 μa V Input Leakage Current All Other Pins I IL -2 +2 μa Input Low Current for I IL1 V IN = 0.4V 170 μa CLOCK SOURCE External Clock Frequency f HFIN -1% 3.6864 +1% MHz VOLTAGE REFERENCE Internal Reference Voltage V REF 1.23 V FSK INPUT Input Voltage Range at 0 V REF V FSK OUTPUT Output Voltage at V OUT AC-coupled max 30k load 400 500 600 mv P-P Frequency of (Note 4) For a mark -1% 1200 +1% For a space -1% 2200 +1% Hz Note 1: Specifications to -40 C are guaranteed by design and are not production tested. Note 2: Active currents are measured when the device is driven by an external clock XCEN = 1 condition. Note 3: Guaranteed by design and not production tested. Note 4: Accuracy is guaranteed based on the external crystal or clock provided. 2
PIN NAME FUNCTION 1, 2 Digital Supply Voltage 3, 9, 16, 17, 18 4 Digital Ground Pin Description Active-Low Reset, Digital Input/Output. This pin includes an internal pullup resistor and is driven low as an output when an internal reset condition occurs. 5 OCD Carrier Detect, Digital Output. A logic-high indicates a valid carrier detection on. OCD = 1 when amplitude is greater than 120mV P-P. OCD = 0 when amplitude is less than 80mV P-P. 6 RTS Request to Send, Digital Input. When set high, the device is put into the demodulator mode. A logic-low puts the device into modulator mode. 7 XTAL1 Crystal Pin or Input for External Clock at 3.6864MHz 8 XTAL2 Crystal Pin or Output of the Crystal Amplifier 10 XCEN External Clock Enable, Digital Input. When set high, this pin allows the user to drive an external clock signal through XTAL1. When in this mode, XTAL2 should be left unconnected. An external crystal must be connected between XTAL1 and XTAL2 when set low. 11 AVDD Analog Supply Voltage 12 13 REF FSK Out, Analog Output. Output of the modulator. Provides a phase-continuous, FSK-modulated output signal (1200Hz and 2200Hz output frequencies) to the 4 20mA current loop interface circuit. Reference, Analog Output. The internal voltage reference is provided as output. This pin must be connected to a 0.1μF capacitor. 14 FSK In, Analog Input. Input for the FSK-modulated HART receive signal from the 4 20mA current loop interface circuit. 15 AGND Analog Ground 19 Digital Data Out, Digital Output. Output from the demodulator. 20 Digital Data In, Digital Input. Input to the modulator. EP Exposed Pad. Should be connected to ground (, AGND). Block Diagram AGND AVDD XTAL1 XTAL2 CRYSTAL OSCILLATOR CLOCK GENERATOR POWER MONITOR V REF 1.23V REF XCEN OCD Rx DEMODULATOR DIGITAL FILTER SAMPLE/HOLD ADC RTS Tx MODULATOR DAC 3
Introduction to HART HART is a backward-compatible enhancement to existing 4 20mA instrumentation networks that allows twoway, half-duplex, digital communication with a microcontroller-based field device. The digital signal is encoded on top of the existing instrumentation signal. Communication is accomplished through a series of commands and responses dependent on the specific protocol and network topology. The does not implement any portion of the communication protocol; it only handles the modulation and demodulation of the encoded information. Digital data is encoded using frequency-shift keying (FSK), which is illustrated in Figure 1. A 1 is identified as a mark symbol and is represented with a center frequency of 1.2kHz. A 0 is identified as a space symbol and is represented with a center frequency of 2.2kHz. This allows a throughput of 1.2kbps, with each symbol occupying an 833µs slot. V 1.2kHz MARK "1" Figure 1. HART FSK Signal 2.2kHz SPACE "0" Functional Description The modem chip consists of a demodulator, carrier detect, digital filter, ADC for input signal conversion, a modulator and DAC for output signal generation, and receive and transmit state machine blocks to perform the HART communication. The Block Diagram illustrates the interface between various blocks of circuitry. The input HART signal s noise interference is attenuated by a one-pole highpass filter that is external to the T chip; the attenuated signal is digitized by the ADC and filtered by the receive state machine. The transmit state machine modulates the input to the HART-compliant signal with the help of the modulator and the DAC. Modulator The modulator performs the FSK modulation of the digital data at the input. The FSK-modulated sinusoidal signal is present at the output as illustrated in Figure 1. The modulator is enabled by RTS being a logic-low. The modulation is done between 1200Hz (mark) or 2200Hz (space) depending on the logic level of the input signal. The modulator preserves a continuous phase when switching between frequencies to minimize the bandwidth of the transmitted signal. Figure 2 illustrates an example waveform of the in modulate mode. The data to be modulated is presented in a UART format (start, 8 data bits, parity, stop bit) at. shows the modulated output. Demodulator The demodulator accepts an FSK signal at the input and reproduces the original modulating signal at the output. The HART signal should be presented as an 11-bit UART character with a start, data, parity, and stop bits for proper operation of the demodulator block. The nominal bit rate of the signal is 1200 bits per second. A simple RC filter is sufficient for anti-aliasing. Figure 3 illustrates an example waveform of the in demodulate mode. Applications Information Figure 4 shows the typical application circuit. As the integrates a digital filter, only a simple passive RC filter is required in front of the ADC. R3 and C3 implement a lowpass filter with a 10kHz cutoff frequency; C2 and R2/R1 implement a highpass filter with a 480Hz cutoff frequency. The resistor-divider formed by R1 and R2 provides an input bias voltage of V REF /2 to the ADC input (R1 = R2). The output DAC provides a sine-wave signal, and C4 provides the AC-coupled signal output from the. The typical value of C4 can be anything greater than 20nF based on the application. Technical Support For technical support, go to http://support.maximic.com/micro. 4
1200bps/833μs START 8-BIT DATA PARITY STOP Figure 2. Actual Modulator Waveform START 8-BIT DATA PARITY STOP 1200bps/833μs ONE UART CHARACTER (START, 8 DATA BITS, PARITY, STOP) Figure 3. Actual Demodulator Waveform 5
3.6864MHz CRYSTAL XCEN OCD 3.6864MHz XTAL1 XTAL2 CRYSTAL OSCILLATOR Rx DEMODULATOR CLOCK GENERATOR DIGITAL FILTER POWER MONITOR AGND V REF 1.23V SAMPLE/HOLD ADC AVDD REF POWER SUPPLY 2.7V TO 3.6V R1 C2 R2 C1 C3 R3 HART IN RTS Tx MODULATOR DAC C4 HART AND 4 20mA OUT MICROCONTROLLER 4 20mA DAC OUTPUT Figure 4. Typical Application Circuit Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE PACKAGE CODE DOCUMENT NO. 20 TQFN T2055+3 21-0140 6
REVISION NUMBER REVISION DATE DESCRIPTION Revision History PAGES CHANGED 0 10/08 Initial release. 1 2/09 In the Electrical Characteristics table, changed the Frequency of parameter units from khz to Hz. Added the EP description to the Pin Description table. 3 2 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 7 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.